gro: fix doxygen
gro: fix doxygen

This patch updates GRO API comments to generate correct
API documentation by doxygen.

Signed-off-by: Jiayu Hu <32e4f0527a0c8fd19a4086e4191a246becffadac@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk